As Sycamore3House suggests, you might have left Bluetooth enabled on your phone, resulting in you receiving a picture intended for someone else.
As Samuraisan suggests, a simple wrong number could explain the odd text. (I've had texts inviting me to wild parties In Brighton. I love texting back to tell them that they've just invited an old fogey in Suffolk, especially when I've had a reply saying that I could come anyway!).
Mistakes do happen. e.g. people post question like this in 'Boxing' for no apparent reason ;-)